1. Field of the Invention
The present invention relates to a wide angle imaging optical system used with a solid state image sensor, and more particularly to an imaging optical system with a small outer diameter and a short total length relative to a maximum image height and to an imaging device using the imaging optical system and an image sensor.
2. Description Related to the Prior Art
Various imaging devices comprising a CCD image sensor or a CMOS image sensor and an imaging optical system are known. The imaging optical system has lens groups and an aperture stop. Wide angle imaging optical systems are commonly used in the imaging devices for monitoring or observation purposes. It is essential for the imaging device incorporated in a mobile terminal such as a mobile phone or a distal end portion of an endoscope to be compact. For example, a nasal endoscope has been put into practical use, instead of a peroral endoscope, to reduce physical stress on a patient when the endoscope is inserted. An outer diameter of the distal end portion of the peroral endoscope is reduced to in the order of 9 mm. An outer diameter of the distal end portion of the nasal endoscope is reduced to less than 6 mm. Accordingly, it is indispensable to downsize the imaging device incorporated in the distal end portion.
A cylindrical rigid tip section is provided in the distal end portion of the endoscope. The length of the rigid tip section is in the order of 20 to 30 mm. A bendable flexible section is provided at the rear end of the rigid tip section. A direction of the rigid tip section is changed by operating an angle knob. The imaging device comprising the image sensor and the imaging optical system is incorporated in the rigid tip section. An image of a region of interest is captured through an objective window in the rigid tip section. The rigid tip section has openings connected to respective ends of a light guide fiber, an air/water tube, a forceps tube, and the like. Through the openings, the region of interest is illuminated, the objective window is washed or dried, and the region of interest is treated or a sample is taken using an appropriate tool.
It is important to shorten the rigid tip section in addition to reduction of its diameter because a longer rigid tip section increases physical stress on a patient and restricts bending of a flexible tube section in a narrow body cavity. To shorten the rigid tip section, it is necessary to make the imaging device, being a major component of the rigid tip section, as thin as possible. The length of the imaging device in an axial direction also needs to be shortened. For example, an imaging optical system for an endoscope disclosed in U.S. Pat. No. 4,674,844 (corresponding to Japanese Examined Patent Application Publication No. 6-48327) is designed on the premise that an image sensor is used. The imaging optical system allows principal rays to be incident obliquely on edges of an image plane. A field angle (2ω) is widened to approximately 100° to 118° while the thickness of the imaging optical system itself is reduced to 3.5-4.7 mm and the total length of the imaging optical system, between the front surface (first surface) of the imaging optical system and the image plane, is reduced to 4.2-5.5 mm.
In the wide angle imaging optical system, light rays before and after passing through the aperture stop are scattered. Hence, in the imaging optical system, the light rays move away from the optical axis at an incident surface proximate to an object and at an exit surface (last surface) proximate to the image plane. Because the outer diameter of the imaging optical system is determined in accordance with an outer diameter of the lens proximate to the object or the lens proximate to the image plane, it is required to reduce the diameters of these lenses to reduce the diameter of the imaging optical system. It is necessary to use the largest image sensor possible for the imaging device to be incorporated in an extremely limited space such as the distal end portion of the endoscope. The image size of the imaging optical system needs to be substantially the same as an effective screen size of the image sensor. To improve the image quality while the diameter of the imaging optical system is reduced, it is advantageous to enlarge the outer diameter of the imaging optical system to an extent not exceeding the image size.
In view of the above, the imaging optical system disclosed in the U.S. Pat. No. 4,674,844 is not suitable for current endoscopes that require reduction in diameter and in length, because the outer diameter is large relative to the maximum image height corresponding to the image size, and the lens thickness (the length of the lenses in the optical axis direction) of the entire optical system is long. The above-described requirements for size reduction also apply to the imaging devices incorporated in thin mobile information terminals (PDA: Personal Digital Assistants), typically, mobile phones.